There is some evidence that vitamin D could be used to reduce breast cancer risk. Randomized controlled trials would provide definitive evidence about this effect. However, trials with breast cancer as outcome are expensive and time-consuming. Use of surrogate outcomes has been advocated to accelerate progress in the identification of interventions that could prevent breast cancer. Mammographic breast density is one of the strongest breast cancer risk indicators and is already used as a surrogate outcome in several breast cancer prevention trials. The aim of this double-blind randomized controlled trial is to determine whether daily oral supplementation with vitamin D3 (1,000, 2,000 or 3,000 IU) over a period of 1 year reduces breast density in premenopausal women compared to placebo. A total of 376 women (94 per arm) who live in Quebec City will be recruited. Showing that vitamin D reduces breast density would provide strong support for the idea that vitamin D can be a safe and inexpensive approach for the prevention of breast cancer.